A NOx reducing catalyst is provided in the exhaust gas passage, and to keep this catalyst within a predetermined temperature range that provides a high NOx cleaning rate, it has been proposed to estimate the temperature of the catalyst and to control the rate at which the exhaust gas flows over the catalyst based on this estimated catalyst temperature.
For example, the disclosure of Laid-open Japanese Patent Application H10-68315 calculates the estimated catalyst temperature Tc based on exhaust gas temperature Tg1 at the catalyst inlet and exhaust gas temperature Tg2 at the catalyst outlet using the following equation:Tc=p×Tg1+q×Tg2 (p and q are coefficients obtained from experimentation).